Changes in the health of metal-contaminated soil before and after stabilization and solidification

Stabilization is popularly employed to remediate metal-contaminated soils. It involves the absorption and precipitation of heavy metals to reduce their solubility, movement characteristics, or risk and toxicity. This study aimed to conduct a soil health assessment to determine changes in the health of metal-contaminated soil before and after the application of five stabilizers (acid mine drainage sludge (AMDS), coal mine drainage sludge (CMDS), steel slag, lime, and cement). Soil health assessment, including three soil functions, namely soil productivity, soil stability, and soil biodiversity, evaluated the physical, chemical, and biological indicators (total 16 indicators). Soil health index (SHI) of soil function was calculated by multiplying each indicator score by the weighting factor of each indicator. Total SHI was obtained by summing the three soil-function SHI. Total SHI of the stabilized and test soils followed the order as control soil (1.90) > heavy metal-contaminated soil (1.55) > CMDS-stabilized soil (1.29) > steel slag-stabilized soil (1.29) > AMDS-stabilized soil (1.26) > cement-stabilized soil (0.74) > lime-stabilized soil (0.67). Total SHI of the initial heavy metal-contaminated soil was evaluated as 'normal', before the stabilizer was applied; however, most of the stabilized soils became 'bad' after application of the stabilizers. Furthermore, soils stabilized by cement and lime showed very poor soil health. The results implied that changes in physical and chemical soil properties occurred due to the disturbance caused by the mixing of stabilizers, and ions eluted from the stabilizers could deteriorate soil health further. The findings indicated that soil treated with stabilizers is not suitable for agricultural purposes. Overall, the study suggested that stabilized soil from metal-contaminated sites should be covered with clean soil or monitored for some time before deciding its future agricultural use.